The reactive stroma plays an important role in tumorigenesis. In order to facilitate recruitment of the microvasculature and migration of tumor cells extracellular matrix[unreadable](ECM) must undergo extensive remodelling. The molecular mechanisms that mediate controlled regulation of ECM turnover in quiescent stroma and are responsible for dysregulation of this process during tumorigenesis are not well defined. There is growing evidence, including recent studies from our laboratory, that the members of the Ets family, Flil and Etsl, are the critical regulators of stromal homeostasis in healthy tissues, as well as contributors to formation of reactive stroma. We hypothesize that Fli1 is involved in maintaining quiescent stroma, which facilitates proper interactions between endothelial and perivascular cells in healthytissue blood vessels. Downregulation of Flil and upregulation of Etsl contributes to neoangiogenesis in breast tumors. Specifically, we hypothesize that the primary role of Ets factors as integrators of the TGF-P/CTGF and Ras-MAPK signalling pathways is in regulation of ECM remodeling. This in turn affects cell migration, cell survival, and recruitment of perivascular cells to the newly formed vessels. To test these hypotheses we propose three Specific Aims. In Aim 1 we will determine the possible role of Flil and Etsl in apoptosis of human microvascular endothelial cells (HVMECs). In Aim 2 we will employ an in vitro organotypic coculture of HVMECs and breast fibroblasts to investigate the roles of Etsl and Flil in capillary network formation. Specifically, the influence of TGF-p, CTGF, and tumor-derived factors will be examined using this 3D model. In Aim 3 we will determine the role of Flil in Neu-induced mammary tumorigenesis using various genetic models with altered Flil expression. These will include an existing Flil+/- mice and conditional tissue-specific Flil deletion mutants in fibroblasts and endothelial cells, which will be generated in this project. The results obtained in 3D coculture experiments and in animal models in vivo will be validated using tissue from patients with breast cancer.